1. Field of the Invention
The present invention relates to an inductor for heating the inner circumference of a hole, and the inductor is used for heating a tube from the inside in order to extract the tube for heat transfer in a boiler, a condenser, and so on from a tube plate, or, for heating a cylinder in a similar manner from the inside in order to perform heat treatment of the inner circumference of a cylinder.
2. Description of the Prior Art
Conventionally, a part which a tube is inserted into and fixed at a tube plate has been heated, and loosened in order to extract an existing tube from the tube plate when the tube is replaced for boilers and condensers, and an inductor which is inserted into the inside of the tube for induction heating of the tube has been well-known (refer to, for example, Japanese Patent Application Laid-Open No. 4-22094, and Japanese Patent Application Laid-Open No. 5-337750). Here, the above loosening is caused by a mechanism in which, as a tube is heated in the first place before the tube plate is done; the tube which is going to thermally expand is prevented from expanding in the diameter by a constraint of the tube plate which has been neither heated yet nor expanded; and, instead, an outside diameter/wall thickness ratio is reduced after deformation by compressive yielding, the outside diameter after cooling becomes smaller than the original one to cause a clearance between the tube and the tube plate. That is, the above-described heating is required to be done rapidly (for example, the rate of heating is 100xc2x0 C./second) so that only the tube becomes red hot before heat is transmitted to the tube plate. A conventional inductor 1 which is used for the above induction heating comprises: a solenoidal coil 2a having a configuration in which a conductive tube 2 such as a copper tube is spirally wound as shown in FIG. 4; a transition conductor 2b extending from one end of the solenoidal coil 2a to a feeding terminal (not shown in the figure) ; and a transition conductor 2c extending from the other end of the coil 2a to another feeding terminal (not shown in the figure) through the inside of the solenoidal coil 2a. Moreover, there has been also known another inductor with a configuration in which a magnetic core 3 is arranged in the solenoidal coil 2a as shown in FIG. 5.
As the induction heating by the inductor arranged in the inside of the tube has a low ratio of magnetic-flux concentration on the tube body, the heating efficiency of the above heating is remarkably low, comparing with that of induction heating by the inductor arranged in the outside of the tube. Thereby, it is actually indispensable to arrange a magnetic core for improving action of electromagnetic induction on the tube body in a case in which rapid heating is required like the above-described heating for extracting the tube. Accordingly, the magnetic core is configured to be arranged as shown in FIG. 5.
However, a heat exchanger tube which is a target tube for the above tube extracting has a small diameter, and there is only a narrow space with an inside diameter of about 20-60 mm in the inside of the tube in many cases. Moreover, the outside diameter of the copper tube forming the solenoidal coil usually is 2 mm or more as the hollow portion of the tube is configured to be a cooling water channel.
Then, the above-described limitation by the size causes the following problems: In the first place, there is an increased risk that the solenoidal coil 2a and the transition conductor 2c come into contact with each other to cause a short circuit. The above-described short circuit is required to be avoided at any cost as the short circuit is an event in the coil in which a large current of hundreds of amperes flows, and, then, insulation coating with high reliability exceeding a usual level, that is, a large cost will be required.
In the second place, the magnetic core 3 is required to be arranged eccentric to the solenoidal coil 2a as shown in FIG. 5. The eccentric arrangement of the magnetic core causes clearance shortage in a specific direction on the tube circumference, and makes smooth processing for extracting the tube difficult, as not-uniform heating temperature in the direction of the tube circumference causes not-equal distribution, in the circumference direction of the tube, of the above deformation by compressive yielding; and, then, loosening which makes the tube cross section become elliptical in shape.
In the third place, the cross-sectional area of the magnetic core 3 becomes about xc2xd or less of the space in the solenoidal coil 2a. At induction heating, the magnetic flux is saturated in many cases as the temperature of the magnetic core considerably rises too, and, a value of the saturation magnetic flux density at the risen temperature becomes several times as small as that of the above density at room temperature. Therefore, it is not easy to secure the above-described and preferable rate of the temperature rise when the cross-sectional area of the magnetic core is small, as there is a tendency that the improved effect of the induction action directly depends on the cross-sectional area of the magnetic core.
The present invention has been made considering the above problems, and the object of the invention is to provide an inductor for heating the inner circumference of a hole, by which there is a small risk of a short circuit in the coil, and induction heating of the inner circumference of a hole and so on can be realized rapidly and uniformly in the circumference direction.
An inductor for heating the circumference of a hole according to the present invention is characterized by a configuration in which the inductor comprises a solenoidal coil and a magnetic core arranged therein; the magnetic core has a cylindrical shape; feeding to one end part of the above solenoidal coil is performed through transition conductor which are arranged in an inserted manner into a hollow portion of the cylindrical magnetic core.
The above-described inductor according to the present invention has the following advantages by the configuration in which the transition conductor is arranged in an inserted manner into the hollow portion of the cylindrical magnetic core:
(1) A risk of a short-circuit between the solenoidal coil and the transition conductor may be avoided.
Here, the volume resistivity of ferrite (MO-Fe2O3type iron oxide) suitable for forming a magnetic core at room temperature is of the order of 100-107 xcexa9cm, which is at least 106 times the order of 10xe2x88x926 xcexa9cm which is the volume resistivity of copper metal forming the coil. Therefore, a short-circuit current at indirect contact between the solenoidal coil and the transition conductor through the magnetic core formed with the above-described ferrite must be about 1/1000 times that of direct contact between the solenoidal-coil and the transition conductor, even if it is assumed that quantity indicating the ease of passage of an electric current for the magnetic core is 10 times that for the coil, in the geometrical factors defined by geometrical shape and size, and the value of the volume resistivity becomes 1/100 time the value at room temperature by the temperature rise of the magnetic core at use. That is, the above-described risk of a short-circuit (overheat damages, spark damages, and so on) may be substantially avoided, as a short-circuit current at indirect contact through the magnetic core becomes only hundreds of milliamperes if a short-circuit current at direct contact is hundreds of amperes
(2) As heating of the tube uniformly in the circumference direction is realized by concentrical arrangement of the magnetic core to the coil, the loosening form without distortion of the cross-sectional shape of the tube may be realized when the inductor is used for extracting the tube.
(3) The cooling effect by flow cooling of the solenoidal coil extends to the magnetic core, and the saturation magnetic flux density is maintained at a higher value, as the cross-sectional area of the magnetic core may be expanded up to about 90 percent of the space in the coil, and, furthermore, the inside and outside surfaces of the magnetic core completely face with the solenoidal coil and the transition conductor, respectively. Accordingly, the above-described improved effect of the induction action may be obtained to the utmost limit by both the above increase in the cross-sectional area of the magnetic core, and the saturation magnetic flux density maintained at a higher value. Thereby, the above-described, and preferable rate of the temperature rise may be secured easily and with the minimum power.
Thus, the above-described problems have been solved by the present invention.